The invention relates to an exposure control device for a camera, and more particularly, to such device for automatically reading or sensing information in the form of a binary code defined on the surface of a film cartridge and representing film speed as such cartridge is loaded into a camera, thereby allowing a voltage corresponding to a particular value of film speed to be established for use in the decision of an exposure level.
An exposure control device which is capable of automatically reading information in the form of a binary code which may be defined by a conductive member printed on the surface of a film cartridge as such cartridge is loaded into a camera is already well known, as disclosed in Japanese Laid-Open Patent Application No. 51,736/1978, for example. However, a conventional exposure control device of this kind utilizes a complex electrical circuit, which resulted in an increased cost of a resulting camera.
For a discussion of this aspect, a reference is made to the drawings. FIG. 1 is a circuit diagram of one form of exposure control device for a camera which is proposed in the prior art. In the exposure control device shown in FIG. 1, a terminal 1 is connected to a source of supply voltage +Vcc, and a light receiving element 2 and an integrating capacitor 3 are connected in series across the terminal 1 and the ground, with the junction between the element 2 and the capacitor 3 being connected to the ground through a switch 4 which remains closed, as shown, during the time a shutter remains closed. This junction is also connected to the non-inverting input terminal of a comparator 5 which is used as decision means to determine a proper amount of exposure. The comparator 5 also has an inverting input terminal which is connected to the junction between a pair of resistors 8, 9 connected in series across the terminal 1 and the ground and forming a voltage divider. The output terminal of the comparator 5 is connected to the terminal 1 through an electromagnet 6 which is effective to control the operation of a shutter. The element 2 may comprise a photoelectric transducer element such as may be formed by CdS (cadmium sulfide) element, for example, which exhibits a varying resistance with the brightness of an object being photographed.
The described device operates as follows: Initially, when a shutter is opened in response to a shutter release operation, the switch 4 is opened. Accordingly, a photocurrent flowing through the element 2 passes through the integrating capacitor 3 to charge it. Before the voltage Vc1 across the capacitor reaches a decision voltage V.sub.E which is determined by the voltage divider resistors 8, 9, the comparator 5 produces an output of a low level (which is hereafter referred to as "L" level) to maintain the electromagnet 6 energized, thus preventing the shutter from being closed. However, when the charged voltage Vc1 reaches the level of the decision voltage V.sub.E, the output from the comparator 5 changes to a high level (hereafter referred to as "H" level), thus deenergizing the electromagnet 6 to allow the shutter to be closed.
When the resistance of the element 2 is represented by R.sub.CdS, the resistance of the element 2 at a particular value of brightness by R.sub.CdSO, an incremental change from a reference brightness by .DELTA.EV and the photoelectric conversion coefficient of the element 2 by .gamma..sub.c, these parameters are related as indicated by the following equality: EQU R.sub.CdS =R.sub.CdSO 2.sup.-.gamma..sbsp.c.sup..DELTA.EV ( 1)
It will be apparent from the equation (1) that a change in the increment .DELTA.EV causes a change in the resistance R.sub.CdS.
In the arrangement of FIG. 1, the voltage Vc1 across the capacitor 3 can be expressed in terms of the capacitance C.sub.1 of the capacitor 3 and the photocurrent i.sub.CdS of the element 2, as follows: ##EQU1## Thus, the exposure time t when the equality Vc1=V.sub.E applies is determined as follows: ##EQU2## The substitution of the equation (1) into the equation (3) yields: ##EQU3##
When films which exhibit different values of film speed are used, it is necessary that the level of the decision voltage which is used for the equality Vc1=V.sub.E must vary depending on such value of film speed. This may be achieved by changing the magnitude of the voltage Vc1 by employing a variable optical stop 7 disposed in front of the element 2 to change the amount of light received or by changing the level of the decision voltage V.sub.E by employing a variable ratio of division by the resistors 8, 9.
Considering a situation that the level of the decision voltage V.sub.E is changed by using a variable ratio formed by the resistors 8, 9 in accordance with a particular value of film speed, the decision voltage V.sub.E can be expressed as follows: ##EQU4## where R.sub.1 and R.sub.2 represent the resistance of the resistors 8, 9, respectively. Assuming that the resistances R.sub.1 and R.sub.2 are related such that R.sub.1 &gt;&gt;R.sub.2, we have ##EQU5## Accordingly, by changing the resistance R.sub.2 of the resistor 9, the decision voltage V.sub.E changes, thereby varying the exposure period t indicated by the equation (4). In order to accommodate for any selected value of film speed through a change in the resistance R.sub.2 of the resistor 9 rather than by the optical stop 7 disposed in front of the light receiving element 2, the resistance R.sub.2 must satisfy the following equation: EQU R.sub.2 =R.sub.02 2.sup.-.gamma..sbsp.c.sup..DELTA.SV ( 7)
where R.sub.02 represents a resistance of the resistor 9 for a standard film speed and can be suitably chosen in the design of the electrical circuit of the exposure control device, and .DELTA.SV a change from the standard film speed. It will be seen that the equation (7) is similar in form to the equation (1). Thus, by changing the resistance R.sub.2 of the resistor 9 as an exponential function of the photoelectric conversion coefficient .gamma..sub.c of the light receiving element 2, the decision voltage V.sub.E varies in accordance with the film speed, thus allowing an accommodation for any value of film speed.
FIG. 2 is a table of binary codes representing various values of film speed which are indicated in known form on a film cartridge. The binary code includes six bits, namely, bit A to bit F. Bits A to E are formed by a conductive area representing "1" or a non-conductive area representing "0" while the bit F comprises a common conductive area. In effect, film speed information is indicated by the five most significant bits A to E of the binary code, and only those bits which represent "1's" are connected to the common bit F. Accordingly, when a film cartridge carrying a binary code which represents a film speed is loaded into a camera, contacts S.sub.A to S.sub.E, which are disposed for reading the respective bits A to E in the binary code are turned on for a "1" bit and turned off for a "0" bit. By way of example, it will be seen from FIG. 2 that for a film speed of ISO 100, the read contacts S.sub.A to S.sub.E are operated such that S.sub.A is off, S.sub.B is on, S.sub.C is off, S.sub.D is on and S.sub.E is off.
Reference to FIG. 2 will indicate that the three most significant bits A, B and C of the binary code define a binary counter which is incremented by one for each 1 EV change in the film speed such as for ISO 25, 50, 100, . . . 1600, 3200, . . . Film speeds which are most frequently used are in a range from ISO 50 to 800. By choosing the resistance R.sub.2 of the resistor 9 for ISO 50 as a reference resistance R.sub.02, for example, the resistances for other values of film speed will be related as follows: ##EQU6## In other words, for those values of film speed in a range from ISO 50 to 800 which vary incrementally by 1 EV, a decision voltage V.sub.E corresponding to a particular value of the film speed will be obtained by choosing the resistance R.sub.2 of the resistor 9 shown in FIG. 1 in accordance with the equations (8). Accordingly, by providing a plurality of resistors having different resistances as indicated by the equations (8) in accordance with various ISO values of the film speed, and selecting one of these resistors for connection between the resistor 8 and the ground, a proper value of the decision voltage V.sub.E can be established. Such selection means may comprise a decoder as shown in FIG. 3, for example.
The decoder shown in FIG. 3 includes a terminal 11 to which the "1" or "0" signal of the bit A in the binary code is applied, a second terminal 12 to which the signal for the bit B is applied, and a third terminal 13 to which the signal for the bit C is applied. As shown, the terminal 11 is directly connected to each first input of AND gates 18, 20, 22, and is also connected through an inverter 14 to each first input of AND gates 17, 19, 21. The terminal 12 is directly connected to each second input of AND gates 19, 20 and is also connected through an inverter 15 to each second input of AND gates 17, 18, 21, 22. The terminal 13 is directly connected to each third input of AND gates 21, 22, and is also connected through an inverter 16 to each third input of AND gates 17 to 20.
When "1" or "0" signals of the bits A to C are applied to the terminals 11 to 13, one of the AND gates 17 to 22 develops an output of "H" level in accordance with the binary code, thus allowing one of the resistances indicated by the equations (8) to be selected.
FIG. 4 shows the electrical circuit of another exposure control device available in the prior art. The electrical circuit shown in FIG. 4 includes an electrical circuit portion comprising the light receiving element 2, integrating capacitor 3, comparator 5, electromagnet 6 and resistor 8 which are connected in the same manner as illustrated in FIG. 1. However, the resistor 9 is replaced by an automatic film speed presetting circuit including NPN transistors 31 to 47, PNP transistors 48 to 53, resistors 55 to 73 and read contacts S.sub.A to S.sub.E, all of which are connected between the non-inverting input of the comparator 5 and the ground, and a manual film speed presetting circuit including resistors 74 to 79 and a changeover switch 80.
When a film cartridge carrying a binary code which represents a film speed is loaded into a camera, it is assured that the read contact S.sub.D associated with the bit D or the read contact S.sub.E associated with the bit E will be turned on, as will be evident from FIG. 2. Accordingly, the transistors 48 and 32 are turned on, whereby the transistor 31 is turned off. This prevents the changeover switch 80 which is disposed to select one of the manual presetting resistors 74 to 79 from being connected to the ground, thus disabling the manual presetting circuit. The read contacts S.sub.A, S.sub.B and S.sub.C will be turned on or off in accordance with the bits A, B and C, whereby one of the transistors 33, 34, 37, 39, 43 and 45 will be turned on to connect one of the resistors 59, 62, 67, 68, 72 and 73 across the non-inverting input of the comparator 5 and the ground. It will be understood that these resistors 59 to 73 represent film speed values of ISO 25, 50, 100, 200, 400 and 800, and have resistances which are chosen in accordance with the equations (8). By way of example, if a film cartridge carries a binary code of "00010" representing ISO 25, only the read contact S.sub.D will be turned on. Where the read contacts S.sub.A, S.sub.B and S.sub.C associated with more significant bits are off, the transistor 33 is turned on to connect the resistor 59. By way of another example, if a film cartridge carries a binary code of "10010" representing ISO 50, the read contacts S.sub.A and S.sub.D will be turned on. When the contact S.sub.A is turned on, the transistor 49 is turned on, whereby the transistors 34 and 35 will be turned on while the transistor 33 will be turned off, thus connecting the resistor 62. By way of a further example, if the binary code is "11010" representing ISO 200, the read contacts S.sub.A, S.sub.B and S.sub.D will be turned on. When both the contacts S.sub.A and S.sub.B are on, the transistors 50 and 51 are turned on, whereby the transistor 36 is turned on, the transistors 37, 38 will be turned off, and the transistors 39 to 41 will be turned on. Accordingly, the base-emitter path of the transistor 33 as well as the base-emitter path of the transistor 34 will be short-circuited, turning the transistors 33, 34 off. Hence, only the resistor 68 will be connected in this instance. In a similar manner, the resistor 72 will be connected for ISO 400 and the resistor 73 will be connected for ISO 800. Finally, the resistor 67 will be connected for ISO 100.
When a film cartridge carrying no binary code is loaded into a camera, all of the read contacts S.sub.A to S.sub.D remain off, and hence the transistors 48, 32 are off to allow the transistor 31 to be turned on, whereby one of the manual presetting resistors 74 to 79 will be selected by the changeover switch 80 to be connected to the ground. The resistors 74 to 79 correspond to ISO 25 to 800, respectively, and have resistances as indicated by the equations (8). In this manner, a film speed from ISO 25 to 800 can be manually selected.
It will be understood from the above description that the device shown in FIG. 4 allows a decision voltage to be established by reading a binary code indicated on the surface of a film cartridge. However, the circuit diagram of FIG. 4 indicates that the device utilizes an increased number of electronic parts such as transistors, disadvantageously requiring a very complex circuit arrangement.